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Visual Impairment :
Special Educational Needs
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The causes of severe visual impairment are many, but fall into five main categories :

Regulation and focusing of light

Light entering the eye needs to be controlled and then focused onto the retina. The cornea should be evenly rounded, but corneal damage can affect the passage of rays into the eye causing a distorted image to form. Clouding of the cornea or the lens will restrict the light entering the eye (imagine looking through frosted glass). The iris regulates the amount of light entering the eye. Very bright light can permanently damage the cells in the retina. Damage to the iris (as in aniridia) or lack of pigmentation (as in albinism) can restrict the iris’ ability to control light, which can result in photophobia. Distortion of the eyeball or lack of elasticity of the lens can result in a blurred image being focused on the retina so the image lacks detail. Short and long sight are common, but usually correctible with glasses. Cells in the vitreous humor, sometimes referred to as floaters, can result in disturbance to the image formed.

Detection of light

Light is detected and converted into nerve impulses in the retina. Rods and cones are sensitive to different wavelengths (colours) of light. The central part of the eye has a high concentration of cones while the peripheral regions have a high concentration of rods. Damage of the rods and cones will affect the amount and type of light detected by the eye. Damage to cones can result in reduced or no colour vision, as in achromatopsia, with a reduction in the clarity and detail of the image, while damage to the peripheral regions of the retina can result in a reduced visual field and night blindness.


Damage or problems with the optic nerves resulting in reduced or no information passing from the retina to the brain. Problems with the optic pathways can affect the transmission of nerve impulses to either of the hemispheres of the brain, resulting in hemianopia


Cortical or cerebral visual impairment (CVI) results from problems with the visual cortexes, with result that the brain is unable to process the information it receives from the eyes, or is unable to interpret the information, for example, is unable to link the information to memory so can not recognise what the image is. CVI is difficult to diagnose as the eyes are sending the information to the brain so there is no obvious or observable reason for the impairment.

You can find out more about CVI at the The Cerebral Visual Impairment Society webiste :

Emotional/psychological (also known as hysterical blindness)

A psychological condition stops the individual from using the information that has been processed by the brain. As with CVI there is no observable reason for the impairment.